1. Field of the Invention
This invention relates to DC-to-DC converters and more particularly to forward type, single-ended converters switching at zero current.
2. Description of the Prior Art
DC-to-DC conversion is commonly realized with converter topologies which may be classified within three families, related to the buck, boost and buck-boost switching regulators. Over the years these families have grown with the addition of new members, conceived to accomplish particular goals. However, not every goal could be accomodated. In particular, the containment of switching losses at high frequencies has proven elusive. Consequently, operation at higher frequencies, which should be beneficial from most points of view, has in the past often entailed a compromise in conversion efficiency, considerable stress on switching elements and high levels of unwanted EMI.
These limitations have often been recognized to motivate resonant conversion techniques in which electrical energy is processed via series resonant circuits. The inherent oscillation properties of these circuits enable one to turn-off switches in series with them, in phase with the vanishing of currents flowing through them. Switches can then be made to revert to a voltage blocking state virtually without energy loss. Unfortunately, inherent oscillations have additional implications which amount to serious drawbacks.
One difficulty derives from the requirement that the resonant circuit possess a high Q, an obvious prerequisite to efficient power processing by such a circuit. This requirement has signified lack of stability, in the form of "run-away" solutions, implying unbounded oscillation amplitudes with potentially catastrophic implications. In the words of F. C. Schwarz (in IEEE Power Electronics Specialists Conference, 1975 Record, pg. 194-204), protection against their insurgence "proved cumbersome and introduced added complexity in the form of added switching elements." The addition of switching elements was later circumvented by a "phase angle control" technique. However this latter approach necessitates a high degree of sophistication in its control circuitry.
In addition to the "ever present danger of runaway conditions", resonant converters have sufferred from the disadvantage of requiring switches capable of blocking voltages of either polarity, such as tyristors. However tyristors are relatively slow devices, unsuitable for high frequency operation; furthermore, during conduction they exhibit a pn diode forward voltage drop leading to substantial power dissipation for low operating voltages. While high frequency operation can be achieved by resorting to bidirectional voltage blocking switches consisting, e.g., of a series connection of a fast recovery rectifier with a fast transistor, the required hardware is more complicated; and it is still power wasteful at low operating voltages where the reduction in switching losses could be offset by increased switch "saturation" losses.
Aside from the technical difficulties listed above, resonant conversion may be objected to on fundamental, conceptual grounds because its inherent energy exchange mechanism violates what one may call the principle of unidirectional energy flow. A conversion topology conceived to optimize efficiency and power density should attempt to incorporate this self-evident principle. Unfortunately, existing topologies which meet this criterion, such as members of the buck or boost families, inevitably suffer from switching loss problems.